The majority of packaging for foodstuffs, snacks and similar products is produced on form/fill/seal bagging machines. The mode of operation of such machines and the structure of films preferably processed on such machines is described, for example, in The Wiley Encyclopedia of Packaging Technology (editors M. Bakker, D. Eckroth; John Wiley & Sons, 1986) and in Nentwig (Joachim Nentwig, Kunststoff-Folien, Carl Hanser Verlag 1994, Munich).
Form/fill/seal bagging machines form a tube from a continuously fed film, introduce he contents therein and, once the film has subsequently been heat sealed on all sides around the contents, separate from the fed end of the tube a bag-shaped container, formed from a continuous section of film and filled with the product. The stated container is referred to below as a tubular bag. For reasons of economy, elevated machine running speeds are advantageous for the performance of these operations. Particular requirements are consequently placed upon the flexible packaging film used.
Moreover, in many cases the contents requires the least possible exchange of the atmosphere present in the package with the ambient air around the package. Thus, for example, in the case of oxygen-sensitive contents, the interior of the package may be provided with an oxygen-depleted atmosphere. The penetration of water vapour may, however, also be disadvantageous in the case of moisture-sensitive goods. In order to maintain the desired atmospheric conditions the packaging film must accordingly, on the one hand provide an elevated diffusion barrier to such unwanted gases while, on the other, have no macroscopic leaks, such as pores, which allow convective air flow into or out of the package.
One essential precondition for elevated packaging speeds on form/fill/seal bagging machines is elevated stiffness of the film used. The modulus of elasticity to DIN EN ISO 527 may be used as a suitable measure of the stiffness of a film. Elevated stiffness is required in order to draw it through the machinery with the least possible deformation despite the elevated forces applied thereto. Thus, for example, deformation of the film in the area to be heat sealed results in irregularities in the sealed seams. Apart from impairing the appearance of the film, this may, under certain circumstances, also result in leakage, for example if creases are included in the heat seal and consequently in a failure of the function of the package formed from the film.
Particular requirements with regard to the thermal softening behaviour of the film apply to so-called horizontal form/fill/seal bagging machines, which heat seal the film in the machine direction by means of a heated, driven pair of rotating rollers, through which the areas of the film to be sealed are passed. Before reaching the heat sealing rollers, the film is appropriately preheated. In the zone between preheating and heat sealing, the film is in a thermally softened state and thus has a particular tendency to be introduced irregularly into the heat sealing rollers, so giving rise to irregular seams. Optimum processability of the film entails a heat sealable layer which melts at low temperatures combined with a supporting layer which is as stiff and dimensionally stable as possible at elevated temperatures.
If the stated machines are shut down temporarily, the sections of film already unwound from the feed roll remain in extended contact with the ambient atmosphere. In many cases, especially if the prevailing climatic conditions are hot and moist, certain films may absorb moisture from the ambient air and not only soften but also, under certain circumstances, curl due to the non-uniform increase in volume of the inner and outer layers, or they may shrink or expand due to moisture absorption or undergo an accompanying structural change, such as post-crystallisation. These changes may have such a serious effect on the functioning of the packaging machine that it is impossible subsequently to process the exposed areas of film after such shut-downs. Film curling may be qualitatively assessed using the measurement method specified in relation to the characterisation of the Examples according to the invention.
For transport purposes the bags produced in the manner described above are conventionally consolidated in a transport package containing a large number of packages. A typical example of such a transport package is a carton made from paperboard. Placing the packages in the transport package and transport itself expose the packaging film to elevated stresses.
This frequently results in the formation of creases and folds in the packages. Vibration during introduction into the transport package and during transport repeatedly exposes the film to forces which are transferred by mechanical impact against the inside of the transport package or, within the transport package, against adjacent packages. Such contact results in particular stress in the area of a fold. This may thus result, after a certain number of such impacts, in local failure of the film at the fold. This gives rise to a pore which, by allowing air exchange and consequent spoilage of the contents, may result in failure of the package. The resistance of a film to such stress will be referred to below as flex crack resistance, which may, for example, be quantified by the measurement method specified in relation to the characterisation of the Examples according to the invention.
For the prior art film structures described below, it is generally the case that, at an identical film thickness, increased stiffness of the film due to appropriate selection of one of the stated materials, or, with an identical material, increased film thickness, both result in reduced flex crack resistance thereof.
For these reasons, the stated packages are conventionally produced using multi-layer films having a layer or sequence of layers which ensures film stability and is conventionally located on the outer side of the film, here denoted the support film, followed by a layer, the primary function of which is to provide adequate adhesion to the single or multi-layer sequence of layers located on the inner side, here denoted in brief as the heat sealable layer.
The thickest possible heat sealable layer is advantageous in order to ensure that sealing of the package is effective around creases too. A thick support film also contributes towards elevated mechanical stability of the film. On the other hand, on economic grounds (firstly due to material costs and secondly due to the required elevated machine running speeds), the films should be as thin as possible. An excessively thick support film furthermore has a particular tendency to fail by flex cracking. Depending upon package size and contents, favourable thickness ratios for flexible films in tubular bag films are around 15 to 25 .mu.m for the support film and 40 to 70 .mu.m for the heat sealable layer.
Unless otherwise stated, the polymers present in the individual layers are described using the abbreviations for plastics to DIN 7728 or ISO 1043-1987 (E).
Melting points are stated below in relation to the value determined to ASTM 3418 using DSC (differential scanning calorimetry) analysis.
In multi-layer structures, the sequence of layers is described by a succession of the abbreviations of the polymers for the corresponding layers separated from each other by oblique slashes. The heat sealable layer is always on the right.
Oriented polymers are preferably used as the support film for packaging oxygen-sensitive goods on form/fill/seal bagging machines, such as polyamide (PA) oriented biaxially or monoaxially in machine direction or biaxially oriented polyethylene terephthalate (PET). The polyamide used is predominantly PA6, i. e. polycaprolactam, but other grades of PA are also used, such as for example PA-MXD6, a polymer of m-xylylenediamine and adipic acid. "Polyamide" is taken to mean in the widest possible sense polymeric compounds which are linked by the acid amide group --NH.CO--(c.f. also Kunststoff-Handbuch, volume VI, Polyamide, Carl Hanser Verlag, Munich, 1966). A distinction is made between two groups of polyamides: those synthesised from one monomer by polycondensation of .omega.-aminocarboxylic acids or polymerisation of the lactams thereof to yield the polyamide 6 type and those produced from two monomers (diamines and dicarboxylic acids) by polycondensation to yield the polyamide 66 type (Gnauck, Frundt, Einstieg in die Kunststoffchemie, Carl Hanser Verlag, Munich, 1991). Polyamides are distinguished by numbers which state the number of C atoms in the starting substance or, in the case of two components, in the diamine (first number) and in the dicarboxylic acid (second number) or by an abbreviation describing the diamine or the dicarboxylic acid (for example PA MXD6 prepared from the diamine m-xylylenediamine and the dicarboxylic acid adipic acid).
Abbrevi- .omega.-Aminocarboxylic Dicarboxylic ation acid or lactams Diamine acid
6 .epsilon.-caprolactam -- --
11 11-aminoundecanoic -- -- acid
12 .epsilon.-laurolactam -- --
66 -- hexamethylenediamine adipic acid
610 -- hexamethylenediamine sebacic acid
6I -- hexamethylenediamine isophthalic acid
MXD6 -- m-xylylenediamine adipic acid
6/66 .epsilon.-caprolactam hexamethylenediamine adipic acid
6/6T .epsilon.-caprolactam hexamethylenediamine terephthalic acid
6I/6T -- hexamethylenediamine isophthalic acid & terephthalic acid
6/6I .epsilon.-caprolactam hexamethylenediamine isophthalic acid
6/66 .epsilon.-caprolactam & -- -- .epsilon.-laurolactam Table explaining polyamide nomenclature
In addition, polymers are used in the support film which exhibit adequate stiffness even without orientation. Ethylene/vinyl alcohol copolymers (EVOH) are in particular here used in conjunction with polyamide, wherein the support film in these cases preferably comprises EVOH coextruded between two PA layers, i.e. the structure PA/EVOH/PA, and the EVOH preferably contains 40 to 85 mol. % vinyl acetate with a minimum degree of saponification of 90%. Apart from the pure materials, blends, for example of PA-MXD6 with PA6, are also used.
In the simplest case, the heat sealable layer consists of a single layer. This layer preferably consists of polyolefins, such as for example polyethylene (LDPE, HDPE) or ethylene/.alpha.-olefin copolymers (LLDPE), produced with conventional Zielger/Natta catalysts or with metallocene catalysts, or of polymers derived from olefins, such as for example ethylene/vinyl acetate copolymers (EVA), ethylene copolymers with unsaturated esters (for example EBA), ethylene copolymers with unsaturated carboxylic acids (for example EAA, EMAA) and ionomers. Blends of the stated classes of substances are also conventional in order to achieve desired combinations of properties. Low density (less than 0.915 g/cm.sup.3) ethylene/.alpha.-olefin copolymers produced using metallocene catalyst technology are in particular suitable as heat sealable layer materials due to the low sealing activation temperature and elevated hot tack thereof.
It is also prior art to provide a multi-layer heat sealable layer. For example in order to optimise costs, the above-stated substances may be arranged in such a manner that the layer on the inner side of the film facing towards the product is distinguished by a particularly rapid onset of sealing and the subsequent layer used for the film core, while not melting until higher temperatures is consequently lower in cost or, due to a higher melt strength makes it possible to produce such a heat sealable layer as a blown film. Coupling polymers from the stated classes of substances or polymers produced on the basis thereof, such as those modified by grafting with anhydride, may optionally also be used. Examples of such structures are the sequences of layers LDPE/EVA or LDPE/EAA/ionomer.
The support film and heat sealable layer are conventionally bonded together by a coupling layer. There is a possibility here of coextruding all the layers of the film together, i.e. bringing together the polymers of these layers as molten streams and allowing them to flow in molten form through a common die. An extrudable coupling agent for bonding the support film to the heat sealable layer is required for this process. Suitable prior art coupling agents are, for example, maleic anhydride-modified polymers from the group comprising LDPE, LLDPE and EVA, but an EAA or EMAA may also be used as a coupling agent.
Films of identical structure may, however, also be produced by extrusion coating, i.e. by application of the heat sealable layer in a molten state onto a previously produced support film, which has already been provided on the side to be coated with the coextruded coupling agent or a primer applied after extrusion.
If the support film and heat sealable layer have previously been produced separately, they may be bonded according to the prior art by using a laminating adhesive. Such adhesives are conventionally long-chain isocyanates and polyols mixed immediately prior to application, which, once applied, cure to yield polyurethanes.
Such coating or lamination may here be performed on the same production machinery on which the support film is also manufactured. This is generally more economically advantageous than previously producing a support film on a separate machine. Where oriented support films are used, however, it is not advantageous to perform such further processing on the machine used for the production of the support film.
In addition to the above-stated polymers, the class of substances comprising aliphatic polyketones has also been known for quite some time. These are strictly linear and alternating polymers of carbon monoxide and at least one ethylenically unsaturated olefin. However, only few applications for the production of films are known for these materials. Known applications solely exploit the good oxygen barrier properties or high frequency heatability of these materials.
In a presentation to Maack Specialty Film '96, J. G. Bonner and A. K. Powell describe a five-layer structure PP/coupling agent/aliphatic polyketone/coupling agent/PP as a film heat sealable on both sides for packaging foodstuffs. This structure is essentially distinguished by the good oxygen barrier thereof. It is, however, fundamentally unsuitable for use on high speed form/fill/seal bagging machines due to the symmetrical structure and high-melting heat sealing materials thereof. WO 8607012 describes a multi-layer laminate comprising at least two different extrudable polymers, wherein at least one ply contains a polyketone, preferably an ethylene/carbon monoxide copolymer, and is bonded to another layer, preferably consisting of a halopolymer. This film is particularly suitable for sealing by high frequency electromagnetic waves. While the halopolymers, such as PVC or PVDC, used in the stated structure may indeed be heated with radio frequency waves, due to foodstuffs legislation and environmental concerns, they are not advantageous for packaging foodstuffs.
U.S. Pat. No. 5,232,786 describes a multi-layer coextruded structure comprising at least one aliphatic polyketone and at least one polyamide, polyvinyl chloride or copolyether-ester. These structures are distinguished by low adhesion of the composite. This property, combined with the deficient heat sealing characteristics of such a composite, would apparently make it unsuitable for use as a highly stressed tubular bag film. U.S. Pat. No. 5,077,385 describes a two layer laminate, in which one of the layers consists of an aliphatic polyketone, preferably a terpolymer of carbon monoxide, ethylene and propylene, and the other layer is formed by a polypropylene or polycarbonate. The essential feature of the disclosed laminate is the production of the polyketone from a melt by cooling at a rate of 1.degree. C. to 20.degree. C. per minute. In this manner, the material achieves good barrier properties for water vapour, oxygen and carbon dioxide. Implementation of such a cooling process in machines for producing a film suitable for processing as a tubular bag would result in extremely long machine residence times and thus in very costly films. The stated multi-layer structure is furthermore not suitable for a rapid sealing film.
Polymer blends based on aliphatic polyketones have also been disclosed in the patent literature. WO 09111470 accordingly describes a homogeneous blend of an ethylene/vinyl alcohol copolymer and a copolymer of ethylene and carbon monoxide. The blend is distinguished by good oxygen barrier properties, heatability by high frequency radiation and, in comparison with the pure copolymer of ethylene and carbon monoxide, improved melt strength and is accordingly used inter alia for processing as a blown film. Such a polymer blend and the use thereof in single or multi-layer films is also described in U.S. Pat. No. 4,965,314. In addition to the above-stated advantages of high frequency heatability and good oxygen barrier properties, a film based on such a polymer blend is additionally distinguished by improved puncture resistance.
WO 09606889 and EP 00669374 also disclose blends of an aliphatic polyketone with a linear low density polyethylene (LLDPE) or a high density polyethylene (HDPE), which are distinguished by improved barrier properties not only for oxygen but also for water vapour and hydrocarbons. Containers and films suitable for receiving foodstuffs produced from these materials are thus also claimed. Since aliphatic polyketones combined with LLDPE or HDPE soften earlier when exposed to heat than the pure copolymer of ethylene and carbon monoxide, such blends are not appropriate for the present application.
The object arose of providing an unoriented multi-layer film having good machine processing characteristics for the production of tubular bags on typical packaging machinery combined with elevated flex crack resistance. If good machine processing characteristics are to be achieved, the film must be rapidly heat sealable on one side when exposed to heat, but must nevertheless have elevated overall stiffness and flatness, even when exposed to heat and moisture. It should also be possible to produce the film efficiently and simply with the fewest possible operations and the film should thus contain no oriented components. The film of the invention is halogen-free.